Generally, a magnetic-type position detector detects a change in the position of a detection object relative to a reference part. The magnetic-type position detector may utilize a magnetic flux generator such as a magnet. For example, a position detector disclosed in a patent document 1 (i.e., Japanese Patent Laid-Open No. JP-A-H08-292004) is configured form a closed magnetic circuit having two magnets and two magnetic flux transmission parts that are disposed on a reference part. In such structure, the two magnets are respectively bound by the ends of the two mutually-facing magnetic flux transmission parts. A flow of spill magnetic fluxes from one transmission part to the other occurs within a gap between the respective ends of the two magnetic flux transmission parts. A magnetic flux density detector is configured to move together with the detection object within the gap between the two magnetic flux transmission parts and to output a detection signal according to the magnetic flux passing therethrough. In such manner, the position detector detects the position of the detection object relative to the reference part based on an output signal that is output from the magnetic flux detector.
In the patent document 1, the position detector is described as being capable of detecting a rotation position of the detection object that is rotated relative to the reference part (refer to FIG. 8 in the patent document). The position detector has two magnetic flux transmission parts formed to extend along a virtual circle that is centered on an axis of rotation of the detection object. Further, the position detector arranges two yokes that are disposed to bind or sandwich the magnetic flux density detector in the radial direction of the virtual circle, for concentrating the flow of the magnetic flux to the magnetic flux density detector in between the two magnetic flux transmission parts.
In this case, each of two yokes has a face which faces a respective magnetic flux transmission part. Each respective magnetic flux transmission part has a planar shape. Further, a yoke-facing face of one of the two magnetic flux transmission parts has a convex shape, while a yoke-facing face of the other one of the two magnetic flux transmission parts has a concave shape. Therefore, when the magnetic flux density detector is positioned at a midpoint between the two magnetic flux transmission parts, a permeance differs in two magnetic paths, that is, between (i) a magnetic path from one transmission part to the yoke and (ii) a magnetic path from the other transmission part to the yoke.
In such configuration, a minimum flux position where the magnetic flux density decreases to a minimum in the gap between two magnetic flux transmission parts along the radial direction of the virtual circle shifts away in the radial direction from a position of the magnetic flux density detector. As a result, a change of the magnetic flux density at the proximity of the density detector according to the shift of the density detector in the radial direction of the virtual circle increases. Therefore, a change of an output signal from the density detector due to the shift of the density detector in the radial direction of the virtual circle may increase. Thus, robustness of the position detector for the position shift of the density detector may deteriorate. That is, in other words, the position detection accuracy for detecting the position of the detection object may be deteriorated in the position detector of the patent document 1.